The present invention is concerned with an Infectious Laryngotracheitis Virus (ILTV) mutant, a recombinant vector molecule comprising ILTV DNA, a host cell transformed with said recombinant vector molecule, a process for the preparation of said ILTV mutant, a cell culture infected with the ILTV mutant, a vaccine derived from the ILTV mutant as well as a process for the preparation of such a vaccine.
Infectious laryngotracheitis (ILT) is a respiratory disease of poultry which has also been reported in pheasants and turkeys. Acute forms of the disease are characterized by signs of respiratory distress accompanied by gasping and expectoration of bloody exudate. In addition, the mucous membranes of the trachea become swollen and hemorrhagic. This epizootic form of the disease spreads rapidly and can affect up to 90-100% of an infected flock. Mortality generally averages between 10 and 20%. Milder forms of the disease are characterized by watery eyes, conjunctivitis, persistent nasal discharge and a reduction in egg production. In the absence of the acute signs of the disease laboratory confirmation must be obtained. Virus can be readily isolated from tracheal or lung tissue and the demonstration of intranuclear inclusion bodies in tracheal or conjunctival tissue is diagnostic of infectious laryngotracheitis virus. In addition, rapid identification can be made with the use of fluorescent antibodies.
The etiological agent of ILT is infectious laryngotracheitis virus (ILTV), an alpha-herpesvirus containing a linear double-stranded DNA genome approximately 150 kilobase pairs (kb) in length. ILTV has been found to exist in two isomeric forms and exhibits the physical characteristics of class 2 herpesviruses such as pseudorabies virus (PRV) and equine herpesvirus 1 (EHV-1).
Control by vaccination of ILTV infection has been a long-sought goal.
In general, attenuated live virus vaccines are preferred because they evoke a more long-lasting immune response (often both humoral and cellular) and are easy to produce.
Presently live infectious laryngotracheitis vaccines are derived from multiple passages in embryonated eggs. Vaccination has generally been used only in areas where the disease is endemic since vaccination can result in the occurence of long term "carrier" birds. Latent infections can be established in the absence of clinical signs and by low levels of virus (450 PFU).
Moreover, although at present animals can be protected against infection of ILTV with live vaccines, these vaccines suffer from a number of drawbacks. These live viral vaccines presently inoculate animals with inadequately attenuated pathogenic virus. Furthermore, because of the attenuation by serial passages uncontrolled mutations are introduced into the viral genome, resulting in a population of virus particles heterogeneous in their virulence and immunizing properties. In addition it is well known that such traditional attenuated live virus vaccines can revert to virulence resulting in disease of the inoculated animals and the possible spread of the pathogen to other animals. In addition, vaccination with existing ILTV vaccine strains seroconverts these animals to positive using ILTV-specific assays such as Elisa's. As a result, these animals can no longer be differentiated from (latent) carriers infected with more virulent field strains of ILTV.
The development of techniques for controlled manipulation of genetic material has allowed the possibility of obtaining attenuated virus vaccines which avoid the disadvantages of the classic attenuated virus vaccines.
It is an object of the present invention to provide an ILTV mutant which can be used for the preparation of a vaccine against ILTV infection, the mutant viruses being attenuated in a controlled way in a manner which excludes the reversion to virulence and which still elicits a strong immune response in host animals.
It is another object of the present invention to provide a mutant ILTV which can be used not only for the preparation of a vaccine against ILTV infection but also against other avian infectious diseases.
It is a further object of the present invention to produce subunit vaccines, pharmaceutical and diagnostic preparations comprising a heterologous polypeptide expressed by an ILTV mutant according to the invention.